1. Field of the Invention
The present invention relates in general to a variable degeneration impedance supply circuit, and an electronic circuit using the same. More specifically, the present invention relates to a variable degeneration impedance supply circuit capable of varying a degeneration impedance using a switch and a capacitor, and an electronic circuit using the same.
2. Description of the Related Art
Advances in electronic technology have led to the development of a variety of types of electronic equipment. Every electronic device has an electronic circuit where passive and active electric elements are combined. Typical examples of electronic circuit are a voltage controlled oscillator (VCO), a frequency divider, etc.
A VCO is a device which makes possible to output a desired oscillating frequency with an externally applied voltage, and frequency divider is a device which divides the frequency of an input signal by a predetermined division rate. The VCO and the frequency divider are useful in many applications such as mobile communication terminals, analog sound synthesizers, etc.
A problem of these electronic circuits is that noise may be inserted into an input signal. To remove the noise, a degeneration impedance circuit combining resistors and capacitors is provided to damp a magnitude of a total output signal.
A cross coupled transistor pair is usually used to supply a degeneration impedance. However, if the magnitude of a degeneration impedance is fixed, it cannot be used for a multi-band frequency signal. For this reason, a related art degeneration impedance supply circuit utilizes a varactor for frequency tuning of an output signal. For instance, in case of a VCO, an oscillating frequency is determined by resonant frequency of parallel combinations of inductors and capacitors. In this case, a varactor is utilized to change the oscillating frequency. This is possible because the varactor has different capacitance values according to an input voltage. Unfortunately, since frequency tuning through a varactor is possible only in a band around a predetermined intermediate frequency, it is difficult to apply the varactor to an electronic circuit operating in a multi-band coverage.